WiMAX (World Interoperability for Microwave Access) is a wireless metropolitan area technique based on the IEEE 802.16, with the signal transmission radius reaching a maximum of 50 kilometers. With the technique of orthogonal frequency division multiplexing (OFDM/OFDMA) which the WiMAX standard adopts, all sub-carriers in an OFDM signal are synchronized in terms of time and frequency. As a result, interference between sub-carriers is brought under control, and inter-carrier interference is suppressed. Moreover, OFDM greatly reduces the protective bandwidth between sub-carriers and enhances the spectrum utilization ratio. All these factors enable the OFDM system to achieve very high cell capacity. The WiMAX standard provides a wireless access alternative solution for services for last mile broadband access, as well as providing backhauling. Compared with other protocols, the protocol supports low latency applications such as voice and video, provides broadband connections both light-of-sight (LOS) and none-LOS (NLOS) between subscriber terminals and the base station (BS) and will support hundreds of subscribers from a single BS. It is a new and promising technique with many advantages for future wireless access networks.
However, WiMAX works at the spectrum ranging from 2 GHz to 11 GHz, and LOS propagation is preferred. As a result, the coverage area is limited, especially in urban areas. At the edge of cells or in shadowing areas, user data throughput drops sharply and some coverage blind areas can be found due to complex wireless environments. In addition, due to the high spectrum, indoor coverage turns to be a key issue and currently there is no effective solution.
Combating these problems by simply increasing the density of BS results in an increase in costs of both devices and network wiring. Therefore, the more practical solutions with low costs are expected.